The present invention relates to a method of controlling automatic focusing for use in a still camera, cinecamera, photomicrography taking device or the like.
There has been developed a plurality of devices for automatically adjusting focus conditions of photographic optical system.
FIG. 1 shows a fundamental construction of such an automatic focus control or adusting device. An image of an object 1 to be photographed is formed on a light receiving unit 3 through an object lens 2, a photoelectrically converted output of the light receiving unit 3 is properly treated in a signal treating device 4 thereby deciding or evaluating focusing conditions of an image formed on the unit 3, that is, front focus, in-focus and rear focus conditions, and the evaluated result is displayed on a display unit 5. If the decided result is the front focus condition or the rear focus condition a lens drive motor 7 is rotated in the given direction through a motor drive unit 6 in accordance with these focus conditions thereby moving the lens 2 in an optical axis direction by 1 pitch, thereafter focusing conditions are again decided thereby automatically controlling the lens 2 at the in-focusing position by repeating the above operations.
As a method of deciding focusing conditions based on the photoelectrically converted output obtained by the light receiving unit 3, for example, there is proposed a method of detecting defocused image which uses an evaluation function for calculaing for example defocused amount of image. In the defocused image detecting method, for example, as the light receiving unit 3 use is made of two arrays of light receiving elements arranged in the front and the rear of conjugate surface to predetermined focal plane of the lens 2 at equidistant position so as to receive the same image, photoelectrically converted outputs of these two photocell arrays are arithmetically operated in accordance with evaluation function thereby obtaining evaluation value representing defocused amount of each image, these evaluation values are compared to decide focusing conditions such as front focusing, in-focusing and rear focusing conditions.
FIG. 2 is a diagram showing construction of conventional optical system in a single-lens reflex camera which adopts such a defocused image detection method. In FIG. 2 the control protion of a quick return mirror 13 arranged in an optical path formed between an objective lens 11 and a film 12 is formed by a half mirror 14, the light beam reflected by the half mirror 14 is led to an inspection optical system comprising a focusing screen 15 and a pentaprism 16, the light beam transmitted through the half mirror 14 is led downwards by a reflecting mirror 17 provided to the rear surface of the quick return mirror 13, and the downward led beam is transmitted through a half mirror 18 to lead on a photocell array 19A and the light beam reflected by the mirror 18 is reflected by a total reflection mirror 20 to lead another photocell array 19B which is placed on same substrate 21 as the photocell array 19A. The photocell arrays 19A, 19B ae displaced in the front and the rear of optically conjugate surface to the film 12 with equidistant position, image of same portions of the object to be taken is projected on both photocell arrays 19A, 19B, the photoelectrically converted output of each photocell array is arithmetically operated according to predetermined evaluation function to obtain evaluation value representing defocused amount of image thereby deciding focusing conditions of front focus, in-focus and rear focus based on the evaluation value. As to evaluation function if the output of ith element in photocell array is X.sub.i, for example, the sum from maximum value of .vertline.X.sub.i -X.sub.i-1 .vertline. to predetermined numberth or the like is used.
FIG. 3 shows a relation between the lens position and respective evaluation values obtained by arithmetically operating the output of photocell arrays 19A and 19B based on the evaluation function. In FIG. 3 a curve F.sub.1 represents an evaluation value of photocell array 19A and a curve F.sub.2 represents an evaluation value of photocell array 19B. Respective evaluation values F.sub.1 and F.sub.2 of photocell arrays 19A and 19B have maximum value when in-focusing condition is obtained just on respective photocell arrays 19A and 19B and the evaluation values of both photocell arrays are equal with each other when in-focusing condition is obtained on the film 12. Respective evaluation values F.sub.1 and F.sub.2, therefore, are obtained at any position of the lens 11, and compared with each other thereby deciding the front focus condition in the case of F.sub.1 &gt;F.sub.2, the in-focusing condition in the case of F.sub.1 =F.sub.2, and the rear focus condition in the case of F.sub.1 &lt;F.sub.2, so that automatic focus adjusting can be performed according to the decided result by moving the lens 11 close to the short distance side (+ direction) in the case of F.sub.1 &gt;F.sub.2 or close to the long distance side .infin. (- direction) in the case of F.sub.1 &lt;F.sub.2.
In the above focus adjusting method, however, focusing conditions F.sub.1 &gt;F.sub.2, F.sub.1 &lt;F.sub.2 can be decided, but the moved distance of the lens cannot be obtained. Considering security of in-focusing precision, therefore, the lens 11 is moved in stepwise with moving amount within focal depth thereby obtaining evaluation values F.sub.1 and F.sub.2 each time resulting in a decision of the focusing condition. This cycle is repeated. However the time required for one decision cycle including stepwise moving time for the lens takes from several tens of ms to several hundreds of ms according to brightness of the object so that when a standard lens of focal length 50 mm, F number 1.4, focal depth 80 .mu.m, and total lens moving distance 7.5 mm is used as the lens 11, if in-focusing position is placed at close distance of 7 mm with lens moving amount from the .infin. side and current lens position is placed at .infin. position, the time required to move the lens 11 to the in-focused position takes (7/0.07).times. 0.1=10 (sec) if lens moving amount of one step is 70 .mu.m. This time is not practically used.